The "check engine" light has been glowing for two weeks. It could be warning you of dire mechanical problems or a plugged air, fuel or emissions filter that will eventually cause problems. Or it could be merely nagging you because the gas cap wasn’t fully tightened the last time you filled up.

Most automotive dealerships will charge a flat fee of $75 or more to read codes, then charge by the hour to fix the problem. Many local auto parts stores read codes for free, then suggest and sell parts related to the problem.

Redfield, Iowa, farmer Dale Meyers chose a third route and bought a diagnostic scan tool. Meyers reads codes, then decides if he can fix problems with his family’s vehicles himself or needs to hand them over to a professional mechanic.

"It cost me around $150, but I’ve more than got my money out of it," Meyers says. "I used it to figure out that a filter on the evaporative system on my Chevy truck’s fuel tank was plugged from driving on gravel roads. It was no big deal to clean that filter myself. I use it to turn off the ‘check engine’ light in my daughter’s TrailBlazer once or twice a year—she doesn’t always get her gas tank cap on tight enough.

"There’s a learning curve to using them, and you have to have some basic mechanical skills to make sense of what they tell you," Meyers says. "But owning my own scan tool has definitely saved me money, compared with running to a dealership every time a warning light comes on."

DIY decision. Entry-level diagnostic code readers can cost from $50 to $100. Code readers come with harnesses that connect to the D-shaped, 16-pin OBD-II diagnostic port on all cars and light/medium duty trucks sold in the U.S. since 1996.

Some code readers perform exactly as their name says: they read codes. They do not offer "live" readings.

Other readers include code definitions and provide freeze-frame data—a snapshot of the vehicle parameters at the exact time the fault code was set.

Most code readers allow users to turn off "check engine" lights after the codes have been read. With the code defined, the vehicle owner can then decide whether to repair the vehicle
or take it to a professional.

Advanced options. Diagnostic scan tools use the same OBD-II connector to read and display codes, but offer expanded options to help users diagnose and repair problems. Prices for diagnostic scan tools start at $125 for a basic unit for do-it-yourself mechanics and exceed thousands of dollars for professional-grade units that come with full-scale diagnostic and monitoring capabilities.

"A good scan tool allows you to look at ‘live’ data," says Joe Ebner, tech support with AutoTap. "It will not only give you a graph of what your vehicle is doing but it will provide an example of what a good reading should look like. Then it will sort of take you by the hand and use a diagnostic flowchart to help you figure out exactly what the problem is."

Jennifer Grabowski, product manager for Actron Diagnostic Scan Tools, says that while many of the codes that trigger a "check engine" light are related to government-mandated emission control systems, codes for anti-lock braking system and transmission-related codes are diagnosed by some scan tools.

"Our newest [scan] tool now includes the top reported fixes for the codes that cause the ‘check engine’ light to illuminate," Grabowski says. "There are more than 3 million fixes in the database of our CP9580 Autoscanner Plus that indicate the most probable solution to the problem specific
to your vehicle and the code. This information points users in the direction to correctly fix their
vehicle. For under $250, the CP9580 will provide all the information most do-it-yourselfers want and need."